Upon power loss of AC power, a conventional storage controller is forced to offload a cache content as quickly and reliably as possible from a cache memory to a local persistent storage device using power from a limited-reserve battery backup unit. The persistent storage device (i) is commonly local to avoid counting on remote devices to be powered up and (ii) utilizes very low amounts of power to avoid large batteries. The very low power results in the persistent storage device having a limited access bandwidth. Large batteries are very expensive and have decreasing reliability over time.